The development of low loss, fused silica lightguide fiber over the last few years has led to the investigation of high temperature (e.g., approximately 2000.degree. C.) heat sources, for the drawing of high strength fiber from a lightguide preform. Of the possible heat sources, the oxy-hydrogen torch, the CO.sub.2 laser and induction and resistance furnaces have been employed for drawing the high silica fibers. The torch method, while inexpensive, cannot maintain a uniform diameter over long lengths of fiber. The CO.sub.2 laser provides the cleanest drawing atmosphere, but requires special optical designs to radially distribute the energy for drawing and is limited in power. Induction furnaces are among the most useful high temperature sources.
An induction furnace is described in an article by R. B. Runk entitled "A Zirconia Induction Furnace for Drawing Precision Silica Waveguides" which was published in the Optical Fiber Transmission II Technical Digest (TuB5-1), Feb. 22-24, 1977. Typically, an induction furnace uses a zirconia susceptor. The susceptor has a tubular shape and acts as a semiconductor when heated to temperatures above 1400.degree. C. After the zirconia susceptor has reached the desired temperature of approximately 2100.degree. C., a glass preform is then introduced into the middle of the susceptor, known as the hot zone, a portion of the preform is reflowed, and lightguide fiber is drawn therefrom.
Induction furnaces with zirconia susceptors, however, are subject to various problems including preheating delays, spalling, fractures and low melting eutectic formation. Newly installed induction furnaces with a zirconia susceptor must be preheated to temperatures of at least 2000.degree. C. It takes numerous hours for the zirconia susceptor to reach such temperatures. Because zirconia acts like an insulator up to temperatures of approximately 1400.degree. C. a pre-heat susceptor such as a graphite slug is placed in the center of the zirconia susceptor. The graphite slug couples to the susceptor, elevating the temperature of the zirconia susceptor above the 1400.degree. C. mark and enabling the susceptor to become a semiconductor instead of an insulator.
In order to preserve fiber strength, it is necessary to prevent contamination of the preform and fiber surfaces. During the first several days of drawing fiber from a furnace with a zirconia susceptor, a lower than average fiber yield is produced. The reason for the reduced quality of fiber produced is due to spalling zirconia particles which deposit on the fiber and/or preform that creates flaws that typically cause fiber breaks. Particles of zirconia can also become separated from the susceptor if cracks develop due to temperature cycling such as during start up, temporary power loss, or aging. As a result of the zirconia particles depositing on the fiber, a significant quantity of the overall fiber drawn from the furnace is not of an acceptable quality.
Once a zirconia susceptor has been heated, the susceptor must be kept above 1400.degree. C. in order to remain coupled. When the furnace is shut down, due to a power failure or draw problems, the zirconia cools through its structural transition, cracks, and must be replaced. Having to replace the zirconia susceptor causes the furnace to be out of operation for an extensive amount of time because the zirconia susceptor must be heated and stabilized. From a manufacturing standpoint it would be beneficial to have the furnaces active for a greater percentage of time than current factors permit.
Another common drawback associated with zirconia susceptors is that furnace failure results when a preform comes in contact with a hot zirconia susceptor. The preform generally sticks to the susceptor wall. Removal can lead to cracking of the zirconia. Additionally, because SiO.sub.2 and ZrO.sub.2 react to form a lower melting eutectic composition, the zirconia susceptor tubes can fail after such contact through the formation of a large defective region. Accordingly, there is a need for an improved induction furnace.